Heating, ventilating, and air conditioning (HVAC) systems have continued to evolve with a focus on greater efficiency, reliability, and greater human comfort. At a most basic level, an HVAC system exists in many homes. Such a basic system typically includes a thermostat, a furnace, and dampers on the various vents located in the rooms of the house. The resident sets a desired temperature on the thermostat, and when the ambient temperature falls below that set temperature, the thermostat commands the furnace to turn on. Individual room temperature control throughout the house is manually regulated by physically opening or closing the vent dampers. When the ambient temperature around the thermostat exceeds the set temperature by a given amount, the furnace is commanded off.
In such a basic on-off control mode, a typical controller could be a bi-metal switching element or a mercury switch activated by a bi-metal mechanism in the thermostat. In this forced air home furnace example, when the temperature is too cold the actuator open line is energized resulting in a valve or damper repositioned to increase heating. As the controlled space temperature increases the element activates the close line closing the damper or valve terminating the heating cycle. This type of control results in 100% or 0% heating and often has relatively large temperature swings.
Better control can be achieved with these simple controllers by adding a heating element with the bimetal and a “hold position” in the output. In the hold position the actuator's open and close control lines are de-energized. By activating the heating element when the actuator begins opening the open run time and the resulting heating effect can be limited. The net result is more even continuous heating.
In more complex HVAC systems, such as for large office buildings, malls, large apartment buildings, etc. such simple single thermostat control of the furnace is no longer practicable. In these larger, more complex installations, the furnace control is typically separate from the damper control, which is automated. That is, in such installations, the furnace may continue to run and provide heat to the entire building, even though a particular office does not require heat. The individual office or zone temperature control is provided by a thermostat that controls the position of the vent damper. When heating is required, the vent damper is commanded to open. When no heating is required, the vent damper is commanded to close.
To achieve more even heating in such installations in the various zones, sophisticated controllers and building automation systems are employed. Such systems use proportional through full proportional/integral/derivative (PID) control strategies. In addition to the PID control loop tuning constants, these controllers have parameters that customize their outputs for the particular actuating device. One type of algorithm that may be employed in these sophisticated controllers is a floating actuator output algorithm. Such an algorithm that generates an open command when the actuator is to be opened, generates a close command when the actuator is to be closed, and generates no command (open and close are floating) to hold the actuator in position.
A floating actuator has two input control signals, namely, drive open and drive close. In general, circuitry for generating and interpretation of such control signals is less costly than that required for analog control signals (e.g. 2V to 10V, 4 mA to 20 mA). Floating input signals are not continuous and higher voltage (usually 24V ac or dc) giving them higher noise immunity than analog control signal. Also, in general, with an analog control system the actuator must “know” what an input of 5V means (i.e. what percent of stroke). This requires a position measuring system within the actuator to achieve the proper commanded position. Further, the on-off-on (open-hold-close) control is more generic/universal than its analog counterpart (0 to 10V, 2 to 10V, 1 to 5 V, 4-20 mA, etc.).
In some cases the signals actually provide power to the motor (3 wire) in others the actuator is powered independently and the input lines are true control lines (4 or 5 wire). The actuator drives open when the open input signal is active and drives close when the close input signal is active. The actuator will hold position if both inputs are active and when no input is active. (Obviously, to hold position, spring return actuators are the independent power type).
When such a floating algorithm is used, the actuator's run time (time required to travel from zero to full stroke) must be entered as part of the control loop setup. This parameter allows the controller to position the actuator based upon the control loop demand signal. The loop is initialized by running the actuator to a known position, usually zero, by applying a close output signal for a time slightly longer than run time. Once the actuator's position is known, the controller keeps track of the time the open and close signals are activated and can calculate the actuators position and the runtime and direction necessary to achieve the desired operating point. Control strategies require precise timing to achieve such control. For example a typical floating control algorithm would, upon initiation/power application, activate the close input signal for a time period slightly longer than the actuator's run time. This action positions the actuator to a known start position (in this case full closed/zero). The control loop demand can be converted to a “drive time” floating control signal based upon the actuator's full stroke run time. For a demand of 50% output, the float open line is activated for one half the actuator's run time. If demand changes to require 60% output, the float open line is again activated for an additional 10% of run time.
When properly setup and tuned the controllers can provide very smooth, even control and rapid response to any temperature disturbance. The goal of such control is to achieve basically a constant temperature at the set point with very little temperature swing. The controllers attempt to achieve this temperature equilibrium by commanding more and finer output changes in the position of the actuator to compensate for the smallest disturbance in the sense temperature in the zone for which it is responsible. These small changes, when translated to the mechanical world of the damper, are often far more effective at wearing things out than making a noticeable control change in the physical position of the actuator.
That is, problems occur with high gain control loops (low throttling ranges) with minimum drive times of less than, e.g., 0.5% of stroke time. For example, with a 2° throttling range (TR) a temperature change of 0.01° would generate a minimum output (0.5%) drive pulse. Since 0.010° is within the noise band of most control systems, an ongoing random series of open and close drive signals are typically generated when at the control setpoint. When the setpoint is changed, a large loop error is generated that results in a large drive time output (1° change would result in a 50% of stroke drive time with a 2° TR). This results in a large change in loop heating (or cooling) which begins to reduce the loop error. As the error is reduced the actuator position is changed with a series of minimum drive time pulses. Although the pulses will trend in a single direction (open or close), because of the signal to noise ratio, control reversals/dithering can still be expected. Over time, this results in unacceptable wear on the actuator. With larger TR's the dithering may be eliminated but the slower changes in loop errors will result in every position change accomplished by a series of minimum drive time signals, which is also unacceptable.
Therefore, there exists a need in the art for an actuator drive that does not allow the floating actuator controller to damage the actuator, but that still provides accurate positioning of the actuator.